Cook the pasta according to the package instructions, then drain. Preheat the oven to 200 degrees, then melt a little butter in a large saucepan. Fry the mushrooms for a few minutes, then drain and set aside. Use a paper towel to wipe the pan.Melt the remaining butter in the pan, then add most of the onions and soften for 1 minute. Add the flour for another minute, then gradually add the milk until you have a smooth sauce. Bring the heat up to make the sauce simmer, stirring for a few minutes to thicken. Turn off the heat, add the ham and almost all the cheese, then season to taste.Place the pasta and mushrooms in a baking dish, then pour the sauce over and mix well in the dish. Sprinkle the rest of the cheese and onions on top, then bake for 10 minutes until golden brown.